In a mobile communication network, mobile stations, such as mobile telephones, handover between base stations, in order to continue communications when the mobile stations are migrating.
When a handover is made while packets are received from a base station, problems, such as packet losses, may occur because radio waves from the base station before handover become weakened, for example.
Accordingly, techniques to prevent packet losses have been proposed. For example, one technique prevents packet losses by instructing mobile stations about a packet boundary which is optimal for switching between base stations (refer to Patent Reference 1).    Patent Reference 1: Japanese Laid-open Patent Publication No. 2003-153327
However, in recent years, mobile stations support web (World Wide Web) accesses, multimedia communications, and the like, and often download larger volume of data from content servers and the like.
In order to download data to a mobile station, data sent from a content server is temporally buffered by a base station or the like, before the data is transferred to the mobile station. This is because the data transfer rate between the content server and the base station or the like, is faster than the transfer rate between base station or the like and the mobile station.
If a handover takes place during downloading of data, data that is not buffered in the base station before the handover and is not yet transferred to the mobile station will not be transferred to the mobile station, since the base stations are switched.
In such a case, after the handover, the data must be downloaded once again from the beginning. In the case of data requiring the real time feature, the buffered data cannot be played back and the playback may be disrupted.